Network Working Group
Internet-Draft TCI
Intended status: Experimental November 25, 2017
Expires: May 29, 2018
Certificate Limitation Policy
draft-belyavskiy-certificate-limitation-policy-05
Abstract
The document provides a specification of the application-level trust
model. Being provided at the application level, the limitations of
trust can be distributed separately using cryptographically protected
format instead of hardcoding the checks into the application itself.
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1. Introduction
Binary trust model standardized as a set of trusted anchors and CRLs/
OCSP services does not cover all corner cases in the modern crypto
world. There is a need in more differentiated limitations. Some of
them are suggested [1] by Google when it limits the usage of
Symantec's certificates. The CRL profile does not fit the purpose of
such limitations. The CRLs are issued by the same CAs that are
subject to be limited.
Currently the set of CAs trusted by OS or browsers can be used for
the validation purposes. In case when a large enough CA becomes
untrusted, it cannot be deleted from the storage of trusted CAs
because it may cause error of validation of many certificates. The
measures usually taken in such cases usually include application-
level limitation of certificates lifetimes, refusing to accept EV-
certificates in other way than DV, requirements to use Certificate
Transparency, etc.
This document suggests a cryptographically signed format dubbed
Certificate Limitation Profile (CLP) designed for description of such
limitations. This format can be used by applications that use
system-wide set of trust anchors for validating purposes or by
applications with own wide enough set of trusted anchors in case when
the trust anchor for the entity found misbehaving cannot be revoked.
Currently the only way to provide such limitations is hard coding
them in application itself. Using of CLPs does not allow to
completely avoid hard coding but allows to hard code only the minimal
set of rarely changing data:
the fact that application uses CLP
the certificate to verify the signature under the CLP file
minimal date of the CLP to be used for the current version of
application.
It will be possible to move the checks for the limitations to the
external cryptographical libraries, such as OpenSSL, instead of
checking them at the application level.
2. Certificate Limitations Profile
A proposed syntax and overall structure of CLP is very similar to the
one defined for CRLs [2].
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CertificateList ::= SEQUENCE {
tbsCertList TBSCertList,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING }
TBSCertList ::= SEQUENCE {
version Version,
signature AlgorithmIdentifier,
issuer Name,
thisUpdate Time,
limitedCertificates SEQUENCE OF SEQUENCE {
userCertificate CertificateSerialNumber,
certificateIssuer Name,
limitationDate Time,
limitationPropagation Enum,
fingerprint SEQUENCE {
fingerprintAlgorithm AlgorithmIdentifier,
fingerprintValue OCTET STRING
} OPTIONAL,
limitations Limitations,
} OPTIONAL,
};
Limitations ::= SEQUENCE SIZE (1..MAX) OF Limitation
Limitation ::= SEQUENCE {
limitID OBJECT IDENTIFIER,
LimitationValue OCTET STRING
-- contains the DER encoding of an ASN.1 value
-- corresponding to the limitation type
-- identified by limitID
}
The ASN.1 format of particular limitations is provided in the
corresponding sections. Anywhere below the Time in ASN.1 notation is
treated according to RFC 5280:
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
2.1. CLP fields
TBD
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2.2. CLP signature
The key used for signing the CLP files should have a special Key
Usage value and/or an Extended Key Usage value.
2.3. CLP entry fields
Each entry in list contains the following fields:
The issuer of the certificate with limited trust.
The serial of the certificate with limited trust.
The fingerprint of the certificate with limited trust (optional).
limitationPropagation. This field indicates whether limitations
are applied to the certificate itself, to all of its descendants
in the chain of trust, or both.
and a subset of the following limitations:
issuedNotAfter - do not trust the certs issued after the specified
date
trustNotAfter - do not trust the certs after the specified date
validityPeriod, days - take minimal value from "native" validity
period and specified in the limitation file
ignoredX509Extensions - list of X.509 extensions of limited
certificate that MUST be ignored for the specified certificate
(e.g. EV-indicating extensions)
requiredX509extensions - list of X.509 extensions that MUST be
present in the certificate to be trusted.
requiredNativeChecking - list of the CA-provided checks that MUST
be applied
applicationNameConstraints - list of domains allowed to be issued
by this certificate
excludedIssueIntermediatory - disallow issuing of the
Intermediatory certificates
The limitations are identified by OIDs
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2.3.1. Limitations
2.3.1.1. issuedNotAfter
When this limitation is present, any certificate matching the entry
and issued after the specified date MUST NOT be trusted
The issuedNotAfter limitation is identified by OID TBA.
issuedNotAfter ::= SEQUENCE {
IssuedNotAfter Time
}
2.3.1.2. trustNotAfter
When this limitation is present, any certificate matching the entry
MUST NOT be trusted after the specified date.
The trustNotAfter limitation is identified by OID TBA.
trustNotAfter ::= SEQUENCE {
TrustNotAfter Time
}
2.3.1.3. validityPeriod
When this limitation is present, no certificate matching the entry
should be treated as valid after specified period from its validFrom.
The validityPeriod is measured in days.
The validityPeriod limitation is identified by OID TBA.
validityPeriod ::= SEQUENCE {
Days INTEGER
}
2.3.1.4. ignoredX509Extensions
When this limitation is present, the extensions listed in this
element should be ignored for the matching certificate.
The ignoredX509Extensions limitation is identified by OID TBA.
ignoredX509Extensions :: = SEQUENCE SIZE (1..MAX) OF ExtenID
ExtenID :: = OBJECT IDENTIFIER
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2.3.1.5. requiredX509extensions
When this limitation is present, the extensions listed in this
element should be present for the matching certificate.
The requiredX509extensions limitation is identified by OID TBA.
requiredX509extensions :: = SEQUENCE SIZE (1..MAX) OF ExtenID
ExtenID :: = OBJECT IDENTIFIER
2.3.1.6. requiredNativeChecking
When this limitation is present, it specifies that the certificates
issued by this CA SHOULD be checked against CRL and/or OCSP,
depending on contents of the extension.
The requiredNativeChecking limitation is identified by OID TBA.
requiredNativeChecking ::= SEQUENCE {
RequiredCRLChecking BOOLEAN,
RequiredOCSPChecking BOOLEAN
}
2.3.1.7. applicationNameConstraints
This limitation are applied like Name Constraints [3] limitation
specified in RFC 5280.
This section implies 2 variants of checks:
The list of names that are allowed for the CA to issue
certificates for
The list of names that are forbidden for the CA to issue
certificates for
The applicationNameConstraints limitation is specified according to
RFC 5280, 4.2.1.10 and reuses OID specified in RFC 5280.
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id-ce-nameConstraints OBJECT IDENTIFIER ::= { id-ce 30 }
NameConstraints ::= SEQUENCE {
permittedSubtrees [0] GeneralSubtrees OPTIONAL,
excludedSubtrees [1] GeneralSubtrees OPTIONAL }
GeneralSubtrees ::= SEQUENCE SIZE (1..MAX) OF GeneralSubtree
GeneralSubtree ::= SEQUENCE {
base GeneralName,
minimum [0] BaseDistance DEFAULT 0,
maximum [1] BaseDistance OPTIONAL }
BaseDistance ::= INTEGER (0..MAX)
2.3.1.7.1. excludedIssueIntermediatory
When this limitation is present, the intermediate certificates issued
by this CA MUST NOT be trusted.
The excludedIssueIntermediatory limitation is identified by OID TBA.
3. Verification of CLP
The verification of CLP SHOULD be performed by the application. The
application should check whether the provided CLP matches the
internal requirements and is correclty signed by the specified key.
4. Verification with CLP
In case of using CLP the checks enforced by CLP should be applied
after the other checks.
The limitation provided by CLP MUST NOT extend the trustworthy of the
checked certificate.
The limitations are applied after cryptographic validation of the
certificate and during building its chain of trust. If the
certificate or any of its ascendants in the chain of trust matches
any record in the CLP, the limitations are applied from the ascendant
to descendants. The issuedNotAfter and trustNotAfter limitations are
applied to find out the actual validity periods for the any
certificate in the chain of trust. If the CLP prescribes to have a
particular extension(s) and the certificate does not have it, the
certificate MUST NOT be trusted.
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Application MAY use more than one CLPs (e.g. app-wide, set of system-
wide, user-defined). When multiple CLPs are in use, the limitations
are applied simultaneously.
In case when more than one chain of trust are valid for a
certificate, if any of this chains is valid after applying the
limitations, the certificate MUST be treated as valid.
5. ASN.1 notation
TBD
6. Security considerations
In case when an application uses CLP, it is recommended to specify
the minimal date of issuing of the CLP document somewhere in code.
It allows to avoid an attack of CLP rollback when the stale version
of CLP is used.
It is recommended to distribute CLPs using the channels that are used
for distribution of the applications themselves to avoid possible DoS
consequences.
7. IANA considerations
TBD
8. Acknoledgements
Special thanks to Rich Salz, Igor Ustinov, Vasily Dolmatov, Stanislav
Smyishlyaev, Patrik Faeltstroem, Alexander Venedioukhin, Artem
Chuprina.
9. References
The current version of the document is available on GitHub
https://github.com/beldmit/clp
10. References
10.1. URIs
[1] https://groups.google.com/a/chromium.org/forum/#!msg/blink-
dev/eUAKwjihhBs/rpxMXjZHCQAJ
[2] https://tools.ietf.org/html/rfc5280#section-5
[3] https://tools.ietf.org/html/rfc5280#section-4.2.1.10
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Author's Address
Dmitry Belyavskiy
Technical Centre of Internet
8 Marta str., 1 bld. 12
Moscow 127083
RU
Email: beldmit@gmail.com
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